1. Field of the Invention
The teachings herein relate to an electrical angle detecting apparatus and method and in particular, to an apparatus and method for detecting the electrical angle of a synchronous motor at zero speed without using commutation sensors.
2. Description of the Prior Art
Electrically commutated synchronous motors are used in various drive applications because of their desirable operating characteristics, such as, for example their high efficiency, high power-to-volume ratio, high torque-to-volume ratio, reliability, robust construction, and quiet operation. The rotor position (i.e., electrical angle) is needed to determine the commutation points necessary for initiating and continuing synchronous motor operation.
Conventional synchronous motor drives rely on rotor angle sensor circuitry such as Hall Effect sensors, resolvers or encoders to detect the rotor angle required for performing the commutation of the phase currents. The use of such feedback devices however presents a number disadvantages in many applications. For example, the use of a resolver or encoders tend to increase the physical size, weight, inertia, and cost of the motor. Furthermore, commutation sensors are prone to failure and decrease the reliability of the motor. Commutation sensors are especially prone to fail when used in operating environments subject to humidity, dust, high temperatures, and submersion.
Therefore, in order to eliminate the need for rotor position sensors, sensorless techniques for ascertaining the rotor position of motors have been developed. However, it is known that accurate, high resolution detection of the rotor position at standstill (i.e., non-rotating, zero speed) using sensorless techniques is difficult and problematic to perform. The results obtained by such techniques are often ambiguous as to the position of the rotor. Problems exist in detecting the rotor position at zero speed because there is no back emf to provide an indication of the commutation points. Furthermore, it is especially difficult to detect the rotor position, at standstill, for a round rotor or non-salient permanent motors.
In order to start a PMSM (permanent magnet synchronous motor) machine from a standstill position at full torque or to guarantee a dither free startup, knowledge of the initial rotor position is required. Thus, the startup of a sensorless drive motor using previously proposed techniques of rotor position detection can result in the motor starting with an undesirable velocity and torque dither due to initial rotor position uncertainty. A dithering in velocity or torque is unacceptable in many drive applications, such as, disk drives, electric propulsion, and high performance servos.
Previously disclosed methods for starting a sensorless permanent magnet synchronous motor (PMSM) include, for example, open loop starting strategies with a fixed PWM pattern as disclosed in xe2x80x9cA Permanent Magnet Motor Drive Without a Shaft Sensorxe2x80x9d, R. Wu, G. Slemon, IEEE Transactions on Industry Applications, vol. 27, no. 5, pp. 1005-1011, 1991. The disclosed open loop method yields to a sensorless controller when a suitable back emf has been developed in the method. With such open loop methods, a full torque and a correct torque polarity cannot be guaranteed at startup due to the lack of back emf at startup. Another sensorless PMSM starting strategy uses forced rotor alignment as disclosed in xe2x80x9cBrushless DC Motor Control Without Position and Speed Sensorsxe2x80x9d, N. Matsui, M. Shigyo, IEEE Transactions on Industry Applications, vol. 28, no. 1, pp. 120-127, 1992. In this disclosed method a dc current is applied to the stator before startup of the motor. The dc current generates a magnetic flux that acts to align the permanent magnet field with the magnetic field generated by the stator excitation due to the applied dc current. In this method, the initial alignment torque is of a polarity determined by the initial position. Thus, a dithering of velocity and torque is associated with this starting method.
Sensorless starting methods applicable to a salient-rotor PMSM have been suggested wherein the rotor position dependent stator inductance is used to obtain position information. Such methods are disclosed in xe2x80x9cA Novel Starting Method of Sensorless Salient-Pole Brushless Motorxe2x80x9d, N. Matsui, T. Takeshita, IEEE Transactions on Industry Applications, pp 386-392, 1994; xe2x80x9cOperation of the Permanent Magnet Synchronous Machine Without a Mechanical Sensorxe2x80x9d, M. Schroedl, IEE Conf. Publication, pp 51-56, 1991; and xe2x80x9cSensorless Torque Control of Salient-Pole Synchronous Motor at Zero Speed Operationxe2x80x9d, T. Aihara, A. Toba, T. Yanase, IEEE Applied Power Electronics Conference and Exposition, vol. 2, pp. 715-720, 1997. These disclosed methods of sensorless starting methods applicable to a salient-rotor PMSM""s use a direct measurement of stator currents to calculate inductance. Accurate determination of the phase inductance is difficult however due to the small signals provided for measurement, the subtransient effect, and the effects of system noise. In addition, such methods are not particularly effective for determining the rotor position in a round rotor machine.
In another standstill, zero speed rotor position detection method, high frequency test currents are injected into the machine under test at standstill as disclosed in xe2x80x9cNew Stand-Still Position Detection Strategy for PMSM Drive Without Rotational Transducersxe2x80x9d, J. S. Kim, S. K. Sul, IEEE Applied Power Electronics Conference, pp 363-369, 1994. This method relies on the presence of a pliable coupling between the rotor and load.
U.S. Pat. No. 5,751,125 discloses a technique wherein the inductance ratio of a delta-wound motor is used to determine the standstill rotor position (electrical angle) of the motor. The disclosed method however yields an ambiguous indication of the rotor angle, since an uncertainty of xc2x1xcfx80 remains in the result of the method. This technique identifies only a position sector in which the rotor angle lies. U.S. Pat. Nos. 5,841,252 and 5,854,548 both disclose rotor position detection methods that use current waveform risetime measurements to obtain the desired rotor position information. These disclosed techniques are ineffective for round rotor machine applications, and also suffer inaccuracies due to measurement noise.
Other previously proposed strategies have suggested a direct measurement of the inductance through an analysis of a current waveform responsive to the applied voltage. Such direct inductance measurement techniques tend to yield poor results due to subtransient effects, and noise. Furthermore, for machines with very little rotor saliency, the current waveform does not change appreciably, and thus cannot be used effectively to indicate the rotor position.
The teachings herein comprise a method and apparatus for determining the position of a rotor by applying a voltage across a pair of phases of a motor and using a voltage measurement responsive to the applied voltage and indicative of the distribution of the phase inductances within the phase pair to which a test voltage is applied to determine the rotor position.
The teachings herein are applicable a wide spectrum motors, including round rotor machines since, as will be discussed in greater detail below, the transient reactance generates a temporary change in the per phase inductance.
The teachings herein, while discussed primarily in the context of accurately detecting the rotor position of a three-phase PMSM at zero speed, can be extended to other poly-phase systems. When starting the PMSM, rotor position is required in order to develop full starting torque and to assure a fast, dither-free startup.